1. Field of the Invention
The invention pertains to the field of metallurgical heat treating, and in particular, to the heat treating of ferrous metal articles under controlled atmospheres. Ferrous metal articles, and in particular, the conventional grades of steel being denoted by grade according to American Iron and Steel Institute (AISI) nomenclature contain carbon. As these articles are raised to elevated temperature for thermal treatment, e.g. hardening, annealing, normalizing and stress relieving, under an ambient furnace atmosphere containing air, hydrogen, water vapor, carbon dioxide, and other chemical compounds the surface of the article will become reactive. It is well-known that the presence of water vapor, hydrogen (H.sub.2), and carbon dioxide (CO.sub.2) in the furnace atmosphere will cause carbon at the surface of the ferrous metal article to react and thus be removed from the article. When the carbon is depleted from the surface of the article, the article no longer has a homogeneous cross section due to the change in chemistry and crystallography thus changing the physical properties such as surface hardness and strength of the finished article. In order to avoid this phenomenon, such articles are heated under a controlled atmosphere containing carbon which is available for reaction with the article being treated, or under an atmosphere that is essentially neutral (to either add a slight amount of carbon to the surface of the ferrous article being heated or prevent removal of carbon from the surface).
Under certain conditions it is desirable to add substantial but controlled amounts of carbon to the surface of the article to increase its surface hardness and wear resistance. This is normally accomplished by heating the article to an elevated temperature (in excess of 690.degree. C.) in a controlled carbonaceous atmosphere that adds a desired percentage by weight of carbon to the surface of the article. In the same manner, if ammonia is added to the controlled carbonaceous atmosphere, nitrogen as well as carbon is added to the surface of the article to produce additional hardness and wear resistance of the surface of the article.
In certain manufacturing operations, it is desirable to remove controlled amounts of carbon from the surface of the article to achieve a predetermined lower percentage of carbon in the surface of the article. This is accomplished by heating the article to an elevated temperature in a controlled carbonaceous atmosphere that removes carbon from the surface of the article.
In its broad aspect then, the present invention pertains to heating ferrous metal articles under an atmosphere which is created to control the surface chemistry of the article being treated.
2. Description of the Prior Art
The prior art is adequately summarized in the section entitled "Furnace Atmospheres and Carbon Control" found at pages 67 through 92, and that portion of the section entitled "Case Hardening of Steel" appearing at pages 93 through 128 of volume 2 of the Metals Handbook published in 1964 by the American Society for Metals, Metals Park, Ohio. The particular volume of the Metals Handbook is referred to as Heat Treating, Cleaning and Finishing. All of the material set forth in the aforementioned sections of the Metals Handbook are incorporated herein by reference and will be referred to from time to time in the specification. In particular, that portion of the section on control of surface carbon content appearing on pages 90 through 91 of the Metals Handbook referred to above, and dealing with the determination of carbon potential of a furnace atmosphere is pertinent to the invention herein disclosed.
As set out in the Metals Handbook, furnace atmospheres such as involved in the instant invention, fall broadly into six groups. The first of these is a so called Exothermic Base Atmosphere which is formed by the partial or complete combustion of a fuel gas/air mixture. These mixtures may have the water vapor removed to produce a desired dew point in the atmosphere.
The second broad category is the Prepared Nitrogen Base Atmosphere which is an exothermic base with carbon dioxide and water vapor removed.
The third broad classification is Endothermic Base Gas Atmospheres. These are formed by partial reaction of a mixture of fuel gas and air in an externally heated catalyst filled chamber.
The fourth broad category is the Charcoal Base Atmosphere which is formed by passing air through a bed of incandescent charcoal.
The fifth broad category is generally designated as Exothermic-Endothermic Base Atmospheres. These atmospheres are formed by complete combustion of a mixture of fuel gas and air, removing water vapor, and reforming the carbon dioxide to carbon monoxide by means of reaction with fuel gas in an externally heated catalyst filled chamber.
The sixth broad category of prepared atmosphere is the Ammonia Base Atmosphere. This atmosphere can be raw ammonia, dissociated ammonia, or partially or completely combusted dissociated ammonia with a regulated dew point.
In-situ generation of carburizing atmosphere in the furnace by decomposition of a hydrocarbon liquid at elevated temperature, is disclosed in U.S. Pat. No. 2,056,175. U.S. Pat. No. 2,161,162 discloses in-situ creation of a carburizing atmosphere in the furnace and use of the spent furnace atmosphere as a carrier gas. U.S. Pat. No. 3,413,161 discloses creation of a carburizing atmosphere by in-situ combustion of a hydrocarbon fuel in the presence of less than stoichiometric amounts of air in the furnace. U.S. Pat. No. 3,620,518 discloses a furnace having a catalytic surface on the furnace walls to create a carburizing atmosphere by the reaction of a hydrocarbon such as butane or propane with air inside the furnace.
Other aspects of carburizing are disclosed in U.S. Pat. Nos. 2,287,651; 2,955,062; 3,356,541 (reissued as RE. 26,935) and U.S. Pat. No. 3,397,875.
U.S. Pat. No. 2,786,003 discloses a method of nitriding a chromium steel by spiking the furnace atmosphere with carbon monoxide to control the depth of nitriding, while U.S. Pat. No. 3,705,053 and U.S. Pat. No. 3,748,195 discloses conventional dissociated ammonia atmosphere nitriding processes wherein oxygen is added to the furnace atmosphere to provide a soft nitrided case. Other aspects of nitriding are disclosed in U.S. Pat. No. 3,892,597.
U.S. Pat. No. 3,519,257 discloses a process for in-situ catalytic generation of a nitriding or carbo-nitriding atmosphere while U.S. Pat. No. 3,663,315 discloses a method of inhibiting soot formation during carburizing. Lastly, U.S. Pat. No. 3,705,058 discloses a method of nitro-carburizing.
All of the foregoing are representative of the state of the art of protective furnace atmospheres, as well as furnace atmospheres for carburizing, decarburizing, carbonitriding or other carbon control in the surface of a ferrous metal article being heat treated.